1. Field of the Invention
The present invention relates to a zoom lens system, an imaging device, and a camera. In particular, the present invention relates to: a zoom lens system which is compact but has a wide view angle at a wide-angle limit and a high zooming ratio, and still is able to perform rapid focusing and has high optical performance particularly in a close-object in-focus condition; an imaging device employing the zoom lens system; and a thin and compact camera employing the imaging device.
2. Description of the Background Art
Size reduction and high performance, particularly, a high zooming ratio, are strongly required of cameras having image sensors performing photoelectric conversion, such as digital still cameras and digital video cameras (simply referred to as digital cameras, hereinafter). In recent years, high-speed focusing and high optical performance in a close-object in-focus condition are increasingly required.
As examples of such zoom lens system having a high zooming ratio, there have conventionally been proposed various kinds of zoom lens systems and imaging optical systems, each having a five-unit configuration of positive, negative, positive, negative, and positive, in which, for example, a first lens unit having positive optical power, a second lens unit having negative optical power, a third lens unit having positive optical power, a fourth lens unit having negative optical power, and a fifth lens unit having positive optical power are arranged in order from the object side to the image side.
Japanese Laid-Open Patent Publication No. 2007-279587 discloses a zoom lens having the five-unit configuration of positive, negative, positive, negative, and positive, in which air spaces between the respective lens units, the first to fifth lens units, are varied to perform magnification change, the interval between the first and second lens units is increased while the interval between the second and third lens units is reduced at a telephoto limit relative to a wide-angle limit, the first lens unit comprises a lens element, the second lens unit includes a positive lens and a negative lens, and the total number of lenses in the first and second lens units is four or less.
Japanese Laid-Open Patent Publication No. 2009-282398 discloses a zoom lens having the five-unit configuration of positive, negative, positive, negative, and positive, in which, in zooming from a wide-angle limit to a telephoto limit, all the lens units are moved so that the interval between the first and second lens units is increased and the interval between the third and fifth lens units is increased, and the relationships among the focal length of the fourth lens unit, the focal length of the fifth lens unit, the focal length of the entire system at a wide-angle limit, and the focal length of the entire system at a telephoto limit are set forth.
Japanese Laid-Open Patent Publication No. 2009-163066 discloses a zoom lens having the five-unit configuration of positive, negative, positive, negative, and positive, in which the intervals between the respective lens units are varied to perform magnification change, the first lens unit comprises a negative lens and at least one positive lens, and the relationship between the refractive index and the Abbe number, to the d-line of the negative lens in the first lens unit, is set forth.
Japanese Laid-Open Patent Publication No. 2009-047785 discloses a zoom lens having the five-unit configuration of positive, negative, positive, negative, and positive, in which, when the lens positions are changed from a wide-angle limit to a telephoto limit, at least the second lens unit moves to the image side, the third lens unit moves to the object side, the fourth lens unit is fixed in the optical axis direction, an aperture diaphragm is arranged close to the object side of the third lens unit, the relationship between the focal length of the second lens unit and the focal length of the fourth lens unit is set forth, and the relationship between the amount of movement of the third lens unit when the lens positions are changed and the focal length of the entire lens system at the telephoto limit is set forth.
Japanese Laid-Open Patent Publication No. 2007-264174 discloses an imaging optical system having the five-unit configuration of positive, negative, positive, negative, and positive, in which the fourth lens unit is fixed relative to the image surface and the intervals between the respective lens units are varied at the time of magnification change from a wide-angle limit to a telephoto limit, the fourth lens unit moves in a direction substantially vertical to the optical axis at the time of image blur compensation, the relationship between the focal length of the first lens unit and the focal length of the entire system at the wide-angle limit is set forth, and the relationship between the focal length of the fourth lens unit and the focal length of the entire system at the telephoto limit is set forth.
Japanese Laid-Open Patent Publication No. 2008-304708 discloses a zoom lens having the five-unit configuration of positive, negative, positive, negative, and positive, in which an optical diaphragm is positioned between the second lens unit and the fourth lens unit, the fourth lens unit comprises a negative lens having at least one aspheric surface and a paraxial radius of curvature of its image side surface, which is smaller than that of its object side surface, and the relationships among the focal length of the fourth lens unit, the focal length of the entire system at a wide-angle limit, and the focal length of the entire system at a telephoto limit are set forth.
The zoom lenses and the imaging optical system, which are disclosed in the above-described patent literatures, are downsized enough to be applicable to thin and compact digital cameras, and have a relatively wide view angle at a wide-angle limit or have a high zooming ratio of about 9 or more. However, these conventional zoom lenses and imaging optical system are not sufficient in the focusing speed and the optical performance in the close-object in-focus condition, and therefore, do not meet the requirements for digital cameras in recent years.